1. Field of the Invention:
The present invention relates to a built-in microphone device for reducing the influence of internal noise of an apparatus in which the built-in microphone device is incorporated.
2. Description of the Related Art
In an audio visual apparatus, such as a video camera, having a built-in main microphone for picking up a sound, internal noise generated by a mechanism section is undesirably received by the main microphone. In order to reduce the influence of such internal noise, a built-in microphone device has been developed. A built-in microphone device includes a noise reference microphone provided in a housing of the apparatus. An internal noise signal which is output from the noise reference microphone is given to an adaptive filter, and the adaptive filter generates a control audio signal. The control audio signal is subtracted from the output signal from the main microphone. Thus, the internal noise is cancelled.
A conventional built-in microphone device operating in this manner will be described with reference to FIGS. 9 and 10. FIG. 9 is a block diagram of a conventional built-in microphone device, and FIG. 10 is a schematic isometric view of the conventional built-in microphone device shown in FIG. 9 and an audio visual apparatus, such as a video camera, in which the built-in microphone device is incorporated. FIG. 10 illustrates the positional relationship between a main microphone 1001 and a noise reference microphone 1005 of the conventional built-in microphone device.
In FIGS. 9 and 10, the main microphone 1001 is provided for picking up an external sound for recording and is provided on an outer surface of a wall of a housing 1010 of the audio visual apparatus. The housing 1010 accommodates a magnetic recording and reproduction section including a tape transfer mechanism and a rotary head. The magnetic recording and reproduction section generates internal noise and is referred to as a mechanism section 1020. The noise reference microphone 1005 is provided in the housing 1010 and is directed toward the mechanism section 1020. The noise reference microphone 1005 picks up internal noise such as sound noise caused by vibration mainly generated from the mechanism section 1020.
An adaptive filter 1030 shown in FIG. 9 identifies a transfer characteristic of internal noise transferred from the noise reference microphone 1005 to the main microphone L001. The adaptive filter 1080 also receives an internal noise signal from the noise reference microphone 1005 and generates a control audio signal based on the internal noise signal. A signal subtraction section 1040 subtracts the control audio signal generated by the adaptive filter 1030 from the output signal from the main microphone 1001. Thus, an audio signal having a reduced internal noise component is output.
The conventional built-in microphone device having ouch a structure operates as follows. The main microphone 1001, which is provided on the wall of the housing 1010. efficiently picks up external sound around the apparatus. Since the mechanism section 1020 operates at this point, internal noise, which should not be picked up, is generated. The internal noise is received by the main microphone 1001 through the housing 1010, as a result of which the signal-to-noise ratio of the sound picked up by the main microphone 1001 is lowered.
The noise reference microphone 1005 captures the internal noise generated by the mechanism section 1020. The adaptive filter 1020 estimates a signal identical with an internal noise signal received by the main microphone 1001 based an the internal noise signal output from the noise reference microphone 1005, and outputs the estimated signal as a control audio signal. The signal subtraction section 1040 subtracts the control audio signal from the output signal from the main microphone 1001, thus removing the internal noise component from the output signal. As a result, an audio signal having a reduced internal noise component is obtained. As an adaptive algorithm used by the adaptive filter 1030, a well known LMS (least means square) algorithm or the like is used.
However, the conventional built-in microphone device having the above-described structure has a problem in that a filter coefficient of the adaptive filter 1030 often is not updated optimally in practical use. For example, the filter coefficient is not converged in the condition of canceling the internal noise, resulting in time-consuming filter coefficient learning. In so-no cases, the filter coefficient is diverged, and thus the internal noise is not sufficiently cancelled.
When one internal noise source is not specified, i.e., when a plurality of internal noise sources are present, there are a plurality of transfer characteristics from the plurality of internal noise sources to the noise reference microphone 1005 and also a plurality of transfer characteristics from the plurality of internal noise sources to the main microphone 1001. Accordingly, the effect of suppressing the internal noise is difficult to obtain.
The conventional built-in microphone device has another problem in that, when the noise reference microphone 1005 picks up the external sound, the built-in microphone device adds an echo to the audio signal. This deteriorates the sound quality. These problems will be described in detail.
(1) When internal noise from the mechanism section 1020 has a sufficiently high sound pressure level, the adaptive filter 1030 accurately estimates (i.e., learns) the transfer characteristic from the noise reference microphone 1005 to the main microphone 1001. However, when the filter coefficient of the adaptive filter 1030 is updated in the, state where the level of the internal noise from the mechanism section 1020 is lower than the level of the external sound or where the operation of the mechanism section 1020 is in pause (i.e., where the level of the internal noise signal from the noise reference microphone 1005 is significantly lower than the level of the output signal from the main microphone 1001), the filter coefficient diverges from a desired characteristic. As a result, the internal noise cannot be cancelled.
(2) In the case where the mechanism section 1020 generating the internal noise operates Intermittently, for example, in the case where recording of video and audio data is started and paused repeatedly in a video camera, an internal noise signal required for learning is not obtained while the apparatus is in a pause. Accordingly, it is difficult to cancel the internal noise from the start of recording of video and audio data.
(3) In the conventional structure, the filter coefficient of the adaptive filter 1030 is converged so as to reproduce the transfer characteristic of the internal noise from the noise reference microphone 1005 to the main microphone 1001. As a result, the internal noise is cancelled. However, when either one or both of the main microphone 1001 and the noise reference microphone 1005 are vibrated, such a vibration acts as a signal disturbing the convergence of the filter coefficient. Then, the filter coefficient of the adaptive filter 1030 does not converge so as to cancel the internal noise. Accordingly, the internal noise is not cancelled.
(4) When internal noise is generated by one mechanism section 1020, the adaptive filter 1030 normally performs the learning operation. However, when there are a plurality of internal noise sources, for example, when the video camera generates a noise of the rotary head and noise created when the lens is zoomed, the following problem occurs. In the case where the noise reference microphone 1005 is located in the vicinity of either one of the internal noise sources, the noise reference microphone 1005 cannot capture the internal noise from the other internal noise source or sources. Even when the noise reference microphone 1005 is located at an equal distance from the plurality of internal noise sources, there are a plurality of transfer characteristic s from the plurality of internal noise sources to the noise reference microphone 1005 and a plurality of transfer characteristics from the plurality of internal noise sources to the main microphone 1001. Accordingly, the effect of reducing the internal noise is difficult to obtain.
(5) When an external audio signal is captured by the noise reference microphone 1005, the audio signal is mixed into the output signal from the main microphone 1001 through the adaptive filter 1030 and the signal subtraction section 1040. As a result, an echo noise is generated, which adversely influences the sound quality.
In one aspect of the invention, a built-in microphone device for use in an apparatus having a mechanism section generating internal noise inside a housing of the apparatus includes a main microphone for picking up an external sound; a noise reference microphone for picking up the internal noise; an adaptive filter member for generating a control audio signal based on an output signal from the noise reference microphone using a filter coefficient; a signal subtraction section for subtracting the control audio signal generated by the adaptive filter member from an output signal from the main microphone to generate a subtraction result; and a filter coefficient update control section for receiving an operation signal generated at the time of an operation of the mechanism section, and in response to the operation signal, updating the filter coefficient of the adaptive filter member based on the subtraction result generated by the signal subtraction section and an output signal from the noise reference microphone.
In one embodiment of the invention, the built-in microphone device further includes comprising a comparison section for determining whether a ratio of a level of the output signal from the noise reference microphone with respect to a level of the output signal from the main microphone is higher than a prescribed threshold value or not. When the filter coefficient update control section receives the operation signal and the comparison section determines that the ratio of the level of the output signal from the noise reference microphone with respect to the level of the output signal from the main microphone is higher than the prescribed threshold value, the filter coefficient update control section updates the filter coefficient of the adaptive filter based on the subtraction result generated by the signal subtraction section and the output signal from the noise reference microphone.
In one embodiment of the invention, the built-in microphone device further includes a comparison section for determining whether a level of the output signal from the main microphone is lower than a prescribed threshold value or not. When the filter coefficient update control section receives the operation signal and the comparison section determines that the level of the output signal from the main microphone is lower than the prescribed threshold value, the filter coefficient update control section updates the filter coefficient of the adaptive filter based on the subtraction result generated by the signal subtraction section and the output signal from the noise reference microphone.
In one embodiment of the invention, the mechanism section is a head moving section of a disk recording apparatus.
In one embodiment of the invention, the mechanism section is a zoom section of a video camera.
In one embodiment of the invention, the mechanism section is an autofocus section of a video camera.
In one embodiment of the invention, the noise reference microphone is provided in the housing and in the vicinity of the main microphone.
In one embodiment of the invention, the built-in microphone device further includes a vibration noise reduction section for maintaining the main microphone and the noise reference microphone in a vibration-free state.
In one embodiment of the invention, the vibration noise reduction section includes a floating section for retaining the main microphone and the noise reference microphone, and a damper section for elastically supporting the floating auction to the housing. The main microphone is directed outward with respect to the floating section, and the noise reference microphone is directed inward with respect to the floating section.
In another aspect of the invention, a built-in microphone device for use in an apparatus having a mechanism section generating internal noise inside a housing of the apparatus includes a main microphone for picking up an external sound; a noise reference microphone for picking up the internal noise; an adaptive filter member for generating a control audio signal based on an output signal from the noise reference microphone using a filter coefficient; a signal subtraction section for subtracting the control audio signal generated by the adaptive filter member from an output signal from the main microphone to generate a subtraction result: and a filter coefficient update control section for, when the mechanism section is operated in a wait state of the built-in microphone device, updating the filter coefficient of the adaptive filter member based on the subtraction result generated by the signal subtraction section and an output signal from the noise reference microphone.
In one embodiment of the invention, the built-in microphone device further includes a comparison section for determining whether a ratio of a level of the output signal from the noise reference microphone with respect to a level of the output signal from the main microphone is higher than a prescribed threshold value or not. When the built-in microphone device is in a wait state and the comparison section determines that the ratio of the level of the output signal from the noise reference microphone with respect to the level of the output signal from the main microphone is higher than the prescribed threshold value, the mechanism section is operated and the filter coefficient update control section updates the filter coefficient of the adaptive filter based on the subtraction result generated by the signal subtraction section and the output signal from the noise reference microphone.
In one embodiment of the invention, the built-in microphone device further includes a comparison section for determining whether a level of the output signal from the main microphone is lower than a prescribed threshold value or not. When the built-in microphone device is in a wait state and the comparison section determines that the level of the output signal from the main microphone is lower than the prescribed threshold value, the mechanism section is operated and the filter coefficient update control section updates the filter coefficient of the adaptive filter based on the subtraction result generated by the signal subtraction section and the output signal from the noise reference microphone.
In still another aspect of the invention, the built-in microphone device for use in an apparatus having a mechanism section generating internal noise inside a housing of the apparatus includes first through nxe2x80x2th main microphones for picking up an external sound; a noise reference microphone for picking up the internal noise; adaptive filter member for generating a control audio signal based on an output signal from the noise reference microphone using a filter coefficient; first through nxe2x80x2th signal subtraction sections respectively for subtracting the control audio signal generated by the adaptive filter member from output signals from the first through nxe2x80x2th main microphones to generate subtraction results; a filter coefficient update control section updating the filter coefficient of the adaptive filter member based on a subtraction result generated by a kxe2x80x2th signal subtraction section and an output signal from the noise reference microphone, so as to reduce the subtraction result, where k is a value among 1 through n; and a directivity synthesis section for receiving the output signals from the first through nxe2x80x2th signal subtraction sections and synthesizing directivities of the first through nxe2x80x2th main microphones.
The first through nxe2x80x2th main microphones are provided in the vicinity of one another, and the noise reference microphone is provided in the vicinity of the first through nxe2x80x2th main microphones inside the housing.
In one embodiment of the invention, the built-in microphone device further includes a comparison section for comparing a level of the output signal from the kxe2x80x2th main microphone and a level of an output signal from the noise reference microphone to generate a comparison result. The filter coefficient update control section updates the filter coefficient based on the comparison result generated by the comparison section.
In yet another aspect of the invention, a built-in microphone device for use in an apparatus having a mechanism section generating internal noise inside a housing of the apparatus includes first through nxe2x80x2th plain microphones for picking up an external sound; a noise reference microphone for picking up the internal noise; an adaptive filter member for generating a control audio signal based on an output signal from the noise reference microphone using a filter coefficients first through nxe2x80x2th signal subtraction sections respectively for subtracting the control audio signal generated by the adaptive filter member from output signals from the first through nxe2x80x2th main microphones to generate subtraction results: an averaging section for calculating an average of the subtraction results generated by the first through nxe2x80x2th signal subtraction sections; a filter coefficient update control section updating the filter coefficient of the adaptive filter member based on the average calculated by the averaging section and an output signal from the noise reference microphone, so as to reduce the average; and a directivity synthesis section for receiving the output signals from the first through nxe2x80x2th signal subtraction sections and synthesizing directivities of the first through ft th main microphones. The first through nxe2x80x2th main microphones are provided in the vicinity of one another, and the noise reference microphone is provided in the vicinity of the first through nxe2x80x2th main microphones inside the housing.
In one embodiment of the invention, the built-in microphone device further includes a comparison section for comparing a level of the output signal from the kxe2x80x2th main microphone and a level of an output signal from the noise reference microphone to generate a comparison result. The filter coefficient update control section updates the filter coefficient based on the comparison result generated by the comparison section.
The present invention functions as follows.
The filter coefficient of the adaptive filter member is updated in response to an operating signal which is generated at the time of an operation of the mechanism section. Accordingly, only when the mechanism section generates internal noise, the filter coefficient of the adaptive filter member is updated and thus appropriately converged so as to cancel the internal noise.
In one embodiment of the invention, the filter coefficient of the adaptive filter member is updated based on the level ratio of an internal noise signal supplied by the noise reference microphone with respect to an output signal supplied by the main microphone or based on the level of the output signal supplied by the main microphone. Thus, the learning operation of the adaptive filter is stabilized.
In one embodiment of the invention, the main microphone and the noise reference microphone are located close to each other. In this manner, the interval between the timing when external sound is picked up by the main microphone and the timing, when external sound is picked up by the noise reference microphone is reduced, so that an echo component is reduced to an audibly negligible level.
In one embodiment of the invention, both the main microphone and the noise reference microphone are maintained in a vibration-free state. Thus, the vibration noise disturbing the learning operation of the adaptive filter member is suppressed and stabilize the learning operation.
When the microphone device is in a wait state (for example, after the power is turned on but before the recording of audio data is started), the mechanism section can be operated to generate internal noise, so that the filter coefficient of the adaptive filter member 1s estimated. Therefore, internal noise generated by a mechanism section operating intermittently can be suppressed from the start of the recording of the audio data.
The first through nxe2x80x2th main microphones and the noise reference microphone are located close to one another, so that the adaptive filter is processed commonly. Accordingly, stereo-type or multiple channel-type microphone devices can be provided without increasing the processing amount. In such a structure, the filter coefficient of the adaptive filter is updated based on the subtraction result from one signal subtraction section and the output signal from the noise reference microphone, or based on the average of the subtraction results from a plurality of signal subtraction sections and the output signal from the noise reference microphone. Accordingly, only the adaptive filter member is required, which simplifies the structure of the microphone device.
Thus, the invention described herein makes possible the advantages of providing a built-in microphone device for allowing a filter coefficient of an adaptive filter to perform a learning operation in a stable manner, so that the filter coefficient converges so as to cancel internal noire generated by an internal noise source in an apparatus in which the built-in microphone device is incorporated.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.